Fiber-based output couplers for wavelength beam combining laser systems

ABSTRACT

In various embodiments, wavelength beam combining laser systems incorporate optical fibers and partially reflective output couplers or partially reflective interfaces or surfaces utilized to establish external lasing cavities.

RELATED APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/051,523, filed Sep. 17, 2014, the entiredisclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to laser systems,specifically wavelength beam combining laser systems having fiber-basedoutput coupling schemes.

BACKGROUND

High-power laser systems are utilized for a host of differentapplications, such as welding, cutting, drilling, and materialsprocessing. Such laser systems typically include a laser emitter, thelaser light from which is coupled into an optical fiber (or simply a“fiber”), and an optical system that focuses the laser light from thefiber onto the workpiece to be processed. The optical system istypically engineered to produce the highest-quality laser beam, or,equivalently, the beam with the lowest beam parameter product (BPP). TheBPP is the product of the laser beam's divergence angle (half-angle) andthe radius of the beam at its narrowest point (i.e., the beam waist, theminimum spot size). The BPP quantifies the quality of the laser beam andhow well it can be focused to a small spot, and is typically expressedin units of millimeter-milliradians (mm-mrad). A Gaussian beam has thelowest possible BPP, given by the wavelength of the laser light dividedby pi. The ratio of the BPP of an actual beam to that of an idealGaussian beam at the same wavelength is denoted M², or the “beam qualityfactor,” which is a wavelength-independent measure of beam quality, withthe “best” quality corresponding to the “lowest” beam quality factor of1.

Wavelength beam combining (WBC) is a technique for scaling the outputpower and brightness from laser diode bars, stacks of diode bars, orother lasers arranged in one- or two-dimensional array. WBC methods havebeen developed to combine beams along one or both dimensions of an arrayof emitters. Typical WBC systems include a plurality of emitters, suchas one or more diode bars, that are combined using a dispersive elementto form a multi-wavelength beam. Each emitter in the WBC systemindividually resonates, and is stabilized through wavelength-specificfeedback from a common partially reflecting output coupler that isfiltered by the dispersive element along a beam-combining dimension.Exemplary WBC systems are detailed in U.S. Pat. No. 6,192,062, filed onFeb. 4, 2000, U.S. Pat. No. 6,208,679, filed on Sep. 8, 1998, U.S. Pat.No. 8,670,180, filed on Aug. 25, 2011, and U.S. Pat. No. 8,559,107,filed on Mar. 7, 2011, the entire disclosure of each of which isincorporated by reference herein.

A variety of WBC techniques have been utilized to form high-power lasersfor a host of different applications, and such techniques often involvethe formation and in-coupling of the resulting multi-wavelength beamsinto optical fibers. However, such schemes are often quite complex andrequire several different optical components for producing themulti-wavelength output beam (via the external WBC cavity), coupling thebeam into the fiber, and aligning resonator modes to the core of thefiber, when some of the resonator modes may be lasing in particularmodes outside the preferred modes of the fiber and/or its core. Thus,there is a need for improved, simpler optical arrangements thatfacilitate the integration of WBC laser systems with output (ordelivery) optical fibers.

SUMMARY

In accordance with embodiments of the present invention, wavelength beamcombining (WBC) laser systems feature multiple emitters (or “beamemitters”), e.g., diode bars or the individual diode emitters of a diodebar, which are combined using a dispersive element to form amulti-wavelength beam. Each emitter in the system individually resonatesand is stabilized through wavelength-specific feedback from a commonpartially reflecting (or “partially reflective”) output coupler that isfiltered by the dispersive element (e.g., a diffraction grating, adispersive prism, a grism (prism/grating), a transmission grating, or anEchelle grating) along the beam-combining dimension. In this manner,laser systems in accordance with embodiments of the present inventionproduce multi-wavelength output beams having high brightness and highpower.

In accordance with various embodiments of the present invention, theoutput (or delivery) fiber, or a portion thereof, is utilized to replacethe discrete output couplers of conventional WBC systems. Thus, aportion of or an interface with the fiber is itself partially reflectiveand utilized, in conjunction with a reflective surface of each beamemitter, to form and stabilize the external WBC laser cavity. Thus,component count and alignment complexity of fiber-coupled WBC lasersystems are reduced, and efficiency of such systems is enhanced. Theoutput coupler / feedback mechanism within the delivery fiber mayinclude or consist essentially of, for example, a fiber Bragg gratingwithin the core of the optical fiber. In other embodiments, a partiallyreflective coating at the input face of the fiber, or between the inputface and an end cap attached thereto, or between two fiber segments,stabilizes the WBC laser via Fresnel reflection of the incoming beams.

Embodiments of the present invention couple the one or more input laserbeams into an optical fiber. In various embodiments, the optical fiberhas multiple cladding layers surrounding a single core, multiplediscrete core regions (or “cores”) within a single cladding layer, ormultiple cores surrounded by multiple cladding layers.

Herein, “optical elements” may refer to any of lenses, mirrors, prisms,gratings, and the like, which redirect, reflect, bend, or in any othermanner optically manipulate electromagnetic radiation. Herein, beamemitters, emitters, or laser emitters, or lasers include anyelectromagnetic beam-generating device such as semiconductor elements,which generate an electromagnetic beam, but may or may not beself-resonating. These also include fiber lasers, disk lasers, non-solidstate lasers, vertical cavity surface emitting lasers (VCSELs), etc.Generally, each emitter includes a back reflective surface, at least oneoptical gain medium, and a front reflective surface. The optical gainmedium increases the gain of electromagnetic radiation that is notlimited to any particular portion of the electromagnetic spectrum, butthat may be visible, infrared, and/or ultraviolet light. An emitter mayinclude or consist essentially of multiple beam emitters such as a diodebar configured to emit multiple beams. The input beams received in theembodiments herein may be single-wavelength or multi-wavelength beamscombined using various techniques known in the art.

Laser diode arrays, bars and/or stacks, such as those described in thefollowing general description may be used in association withembodiments of the innovations described herein. Laser diodes may bepackaged individually or in groups, generally in one-dimensionalrows/arrays (diode bars) or two dimensional arrays (diode-bar stacks). Adiode array stack is generally a vertical stack of diode bars. Laserdiode bars or arrays generally achieve substantially higher power, andcost effectiveness than an equivalent single broad area diode.High-power diode bars generally contain an array of broad-area emitters,generating tens of watts with relatively poor beam quality; despite thehigher power, the brightness is often lower than that of a broad arealaser diode. High-power diode bars may be stacked to produce high-powerstacked diode bars for generation of extremely high powers of hundredsor thousands of watts. Laser diode arrays may be configured to emit abeam into free space or into a fiber. Fiber-coupled diode-laser arraysmay be conveniently used as a pumping source for fiber lasers and fiberamplifiers.

A diode-laser bar is a type of semiconductor laser containing aone-dimensional array of broad-area emitters or alternatively containingsub arrays containing, e.g., 10-20 narrow stripe emitters. A broad-areadiode bar typically contains, for example, 19-49 emitters, each havingdimensions on the order of, e.g., 1 μm×100 μm. The beam quality alongthe 1 μm dimension or fast-axis is typically diffraction-limited. Thebeam quality along the 100 μm dimension or slow-axis or array dimensionis typically many times diffraction-limited. Typically, a diode bar forcommercial applications has a laser resonator length of the order of 1to 4 mm, is about 10 mm wide and generates tens of watts of outputpower. Most diode bars operate in the wavelength region from 780 to 1070nm, with the wavelengths of 808 nm (for pumping neodymium lasers) and940 nm (for pumping Yb:YAG) being most prominent. The wavelength rangeof 915-976 nm is used for pumping erbium-doped or ytterbium-dopedhigh-power fiber lasers and amplifiers.

A diode stack is simply an arrangement of multiple diode bars that candeliver very high output power. Also called diode laser stack, multi-barmodule, or two-dimensional laser array, the most common diode stackarrangement is that of a vertical stack which is effectively atwo-dimensional array of edge emitters. Such a stack may be fabricatedby attaching diode bars to thin heat sinks and stacking these assembliesso as to obtain a periodic array of diode bars and heat sinks There arealso horizontal diode stacks, and two-dimensional stacks. For the highbeam quality, the diode bars generally should be as close to each otheras possible. On the other hand, efficient cooling requires some minimumthickness of the heat sinks mounted between the bars. This tradeoff ofdiode bar spacing results in beam quality of a diode stack in thevertical direction (and subsequently its brightness) is much lower thanthat of a single diode bar. There are, however, several techniques forsignificantly mitigating this problem, e.g., by spatial interleaving ofthe outputs of different diode stacks, by polarization coupling, or bywavelength multiplexing. Various types of high-power beam shapers andrelated devices have been developed for such purposes. Diode stacks mayprovide extremely high output powers (e.g. hundreds or thousands ofwatts).

In an aspect, embodiments of the invention feature a laser system thatincludes or consists essentially of an array of beam emitters eachemitting a beam, focusing optics for focusing the beams toward adispersive element, a dispersive element for receiving and dispersingthe focused beams, thereby forming a multi-wavelength beam, an opticalfiber for receiving the multi-wavelength beam, and a fiber Bragg gratingdisposed within the optical fiber. The fiber Bragg grating receives themulti-wavelength beam, reflects a first portion thereof back toward thedispersive element, and transmits a second portion thereof as an outputbeam composed of multiple wavelengths.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The dispersive element may include orconsist essentially of a diffraction grating (e.g., a transmissivediffraction grating or a reflective diffraction grating). The focusingoptics may include or consist essentially of one or more cylindricallenses, one or more spherical lenses, one or more spherical mirrors,and/or one or more cylindrical mirrors. A portion of the optical fibermay be disposed within an external lasing cavity established between thearray of beam emitters and the fiber Bragg grating. The optical fibermay include or consist essentially of one or more cores at leastpartially surrounded by one or more cladding layers. At least a portionof the fiber Bragg grating may be disposed within at least one core ofthe optical fiber. A first end cap may be attached to the optical fiberand disposed optically upstream of the fiber Bragg grating. A second endcap may be attached to the optical fiber and disposed opticallydownstream of the fiber Bragg grating.

In another aspect, embodiments of the invention feature a laser systemthat includes or consists essentially of an array of beam emitters eachemitting a beam, focusing optics for focusing the beams toward adispersive element, a dispersive element for receiving and dispersingthe focused beams, thereby forming a multi-wavelength beam, an opticalfiber for receiving the multi-wavelength beam, and a partiallyreflective interface disposed within or on the optical fiber. Thepartially reflective interface receives the multi-wavelength beam,reflects a first portion thereof back toward the dispersive element, andtransmits a second portion thereof as an output beam composed ofmultiple wavelengths.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The dispersive element may include orconsist essentially of a diffraction grating (e.g., a transmissivediffraction grating or a reflective diffraction grating). The focusingoptics may include or consist essentially of one or more cylindricallenses, one or more spherical lenses, one or more spherical mirrors,and/or one or more cylindrical mirrors. An end cap may be attached tothe optical fiber. The partially reflective interface may be disposedbetween the end cap and the optical fiber (e.g., where the end cap andthe optical fiber contact each other). The end cap may include, consistessentially of, or consist of a first material. The optical fiber mayinclude, consist essentially of, or consist of a second material. Thefirst material may be different from the second material. The firstmaterial may include, consist essentially of, or consist of sapphire.The second material may include, consist essentially of, or consist ofglass or fused silica. The partially reflective interface may include,consist essentially of, or consist of a coating applied to at least aportion of a surface of the end cap and/or at least a portion of asurface of the optical fiber. The partially reflective interface may bedisposed between a portion of the optical fiber and a void (e.g., abubble) disposed within the optical fiber. The void may include, consistessentially of, or consist of air or vacuum. The optical fiber mayinclude or consist essentially of first and second optical fiberportions joined together. The partially reflective interface may bedisposed between the first and second optical fiber portions. The firstand second optical fiber portions may have different numerical aperturesand/or different core diameters and/or different mode fields. A modefield adaptor or a fiber taper may be disposed optically downstream ofthe partially reflective interface. A mode field adaptor or a fibertaper may be disposed between the first and second optical fiberportions. The first optical fiber portion may include a core regionoptically upstream of the partially reflective interface. The secondoptical fiber portion may include a core region optically downstream ofthe partially reflective interface. A diameter (or lateral dimension) ofthe core region of the second optical fiber portion may be differentfrom (e.g., larger than) a diameter (or lateral dimension) of the coreregion of the first optical fiber portion.

In yet another aspect, embodiments of the invention feature a lasersystem that includes or consists essentially of an array of beamemitters each emitting a beam, focusing optics for focusing the beamstoward a dispersive element, a dispersive element for receiving anddispersing the focused beams, thereby forming a multi-wavelength beam, afirst optical fiber for receiving the multi-wavelength beam, a secondoptical fiber disposed optically downstream of the first optical fiber,and a partially reflective output coupler disposed between the first andsecond optical fibers. The partially reflective output coupler receivesthe multi-wavelength beam, reflects a first portion thereof back towardthe dispersive element, and transmits a second portion thereof as anoutput beam composed of multiple wavelengths.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The dispersive element may include orconsist essentially of a diffraction grating (e.g., a transmissivediffraction grating or a reflective diffraction grating). The focusingoptics may include or consist essentially of one or more cylindricallenses, one or more spherical lenses, one or more spherical mirrors,and/or one or more cylindrical mirrors. The first optical fiber may bedisposed within an external lasing cavity formed between the array ofbeam emitters and the partially reflective output coupler. A first endcap and/or a first focusing lens may be disposed between the firstoptical fiber and the partially reflective output coupler. A second endcap and/or a second focusing lens may be disposed between the partiallyreflective output coupler and the second optical fiber.

In another aspect, embodiments of the invention feature a laser systemthat includes or consists essentially of an array of beam emitters eachemitting a beam, focusing optics for focusing the beams toward adispersive element, a dispersive element for receiving and dispersingthe focused beams, thereby forming a multi-wavelength beam, an opticalfiber for receiving the multi-wavelength beam, and a partiallyreflective output coupler disposed optically downstream of the opticalfiber. The partially reflective output coupler receives themulti-wavelength beam, reflects a first portion thereof back toward thedispersive element, and transmits a second portion thereof as an outputbeam composed of multiple wavelengths.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The dispersive element may include orconsist essentially of a diffraction grating (e.g., a transmissivediffraction grating or a reflective diffraction grating). The focusingoptics may include or consist essentially of one or more cylindricallenses, one or more spherical lenses, one or more spherical mirrors,and/or one or more cylindrical mirrors. The optical fiber may bedisposed within an external lasing cavity formed between the array ofbeam emitters and the partially reflective output coupler. The partiallyreflective output coupler may be disposed within a laser processing headdetachable from at least a portion of the optical fiber and/or from aportion of the laser system that includes the diffractive element and/orthe array of beam emitters. One or more focusing lenses may be disposedwithin the laser processing head. One or more focusing lenses may bedisposed optically downstream of the optical fiber and opticallyupstream of the partially reflective output coupler.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “substantially” and “approximately” mean ±10%, and in someembodiments, ±5%. The term “consists essentially of” means excludingother materials that contribute to function, unless otherwise definedherein. Nonetheless, such other materials may be present, collectivelyor individually, in trace amounts. Herein, the terms “radiation” and“light” are utilized interchangeably unless otherwise indicated. Herein,“downstream” or “optically downstream,” is utilized to indicate therelative placement of a second element that a light beam strikes afterencountering a first element, the first element being “upstream,” or“optically upstream” of the second element.

Herein, “optical distance” between two components is the distancebetween two components that is actually traveled by light beams; theoptical distance may be, but is not necessarily, equal to the physicaldistance between two components due to, e.g., reflections from mirrorsor other changes in propagation direction experienced by the lighttraveling from one of the components to the other.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A is a schematic of a wavelength beam combining (WBC) method alongthe array dimension of a single row of emitters in accordance withembodiments of the invention;

FIG. 1B is a schematic of a WBC method along the array dimension of atwo-dimensional array of emitters in accordance with embodiments of theinvention;

FIG. 1C is a schematic of a WBC method along the stack dimension of atwo-dimensional array of emitters in accordance with embodiments of theinvention;

FIG. 2 is a schematic showing the effects of smile in a WBC method alongthe stack dimension of a two-dimensional array of diode laser emittersin accordance with embodiments of the invention;

FIG. 3A is a schematic of a WBC system including an optical rotatorselectively rotating a one-dimensional array of beams in accordance withembodiments of the invention;

FIG. 3B is a schematic of a WBC system including an optical rotatorselectively rotating a two-dimensional array of beams in accordance withembodiments of the invention;

FIG. 3C is a schematic of a WBC system including an optical rotatorselectively reorienting a two-dimensional array of beams in accordancewith embodiments of the invention;

FIG. 3D illustrates output profile views of the system of FIG. 3C withand without an optical rotator in accordance with embodiments of theinvention;

FIGS. 4A-4C illustrate examples of optical rotators in accordance withembodiments of the invention;

FIGS. 5A-5C illustrate related methods for placing combining elements togenerate one-dimensional or two-dimensional laser elements;

FIG. 6 illustrates a WBC embodiment having a spatial repositioningelement in accordance with embodiments of the invention;

FIG. 7 illustrates an embodiment of a two-dimensional array of emittersbeing reconfigured before a WBC step and individual beam rotation afterthe WBC step in accordance with embodiments of the invention;

FIG. 8 illustrates the difference between slow and fast WBC inaccordance with embodiments of the invention;

FIG. 9A illustrates embodiments using an optical rotator before WBC inboth a single and stacked array configurations in accordance withembodiments of the invention;

FIG. 9B illustrates additional embodiments using an optical rotatorbefore WBC in accordance with embodiments of the invention;

FIG. 10 is illustrative of a single semiconductor chip emitter inaccordance with embodiments of the invention;

FIGS. 11A and 11B are schematics of optical fibers in accordance withembodiments of the invention;

FIG. 11C is a schematic of a WBC laser system in accordance withembodiments of the invention;

FIGS. 12A and 12B are schematics of optical fibers in accordance withembodiments of the invention;

FIG. 12C is a schematic of a WBC laser system in accordance withembodiments of the invention;

FIG. 13 is a schematic of an optical fiber assembly in accordance withembodiments of the invention;

FIGS. 14A and 14B are schematics of optical fibers in accordance withembodiments of the invention; and

FIG. 15 is a schematic of a WBC laser system in accordance withembodiments of the invention.

DETAILED DESCRIPTION

Aspects and embodiments relate generally to the field of scaling lasersources to high-power and high-brightness using an external cavity and,more particularly, to methods and apparatus for external-cavity beamcombining using both one-dimensional or two-dimensional laser sources.In one embodiment the external cavity system includes one-dimensional ortwo-dimensional laser elements, an optical system, a dispersive element,and a partially reflecting element. An optical system is one or moreoptical elements that perform two basic functions. The first function isto overlap all the laser elements along the beam combining dimensiononto a dispersive element. The second function is to ensure all theelements along the non-beam combining dimension are propagating normalto the output coupler. In various embodiments, the optical systemintroduces as little loss as possible. As such, these two functions willenable a single resonance cavity for all the laser elements.

In another embodiment the WBC external cavity system includes wavelengthstabilized one-dimensional or two-dimensional laser elements, an opticalsystem, and a dispersive element. One-dimensional or two-dimensionalwavelength stabilized laser elements, with unique wavelength, can beaccomplished using various means such as laser elements with feedbackfrom wavelength chirped Volume Bragg grating, distributed feedback (DFB)laser elements, or distributed Bragg reflector (DBR) laser elements.Here the main function of the optical system is to overlap all the beamsonto a dispersive element. When there is no output coupler mirrorexternal to the wavelength-stabilized laser element, having parallelbeams along the non-beam-combining dimension is less important. Aspectsand embodiments further relate to high-power and/or high-brightnessmulti-wavelength external-cavity lasers that generate an overlapping orcoaxial beam from very low output power to hundreds and even tomegawatts of output power.

In particular, aspects and embodiments are directed to a method andapparatus for manipulating the beams emitted by the laser elements ofthese external-cavity systems and combining them using a WBC method toproduce a desired output profile. Wavelength beam combining methods havebeen developed to combine asymmetrical beam elements across theirrespective slow or fast axis dimension. One advantage of embodiments ofthe present invention is the ability to selectively-reconfigure inputbeams either spatially or by orientation to be used in slow and fastaxis WBC methods, as well as a hybrid of the two. Another advantage isthe ability to selectively-reconfigure input beams when there is afixed-position relationship to other input beams.

FIG. 1A illustrates a basic WBC architecture. In this particularillustration, WBC is performed along the array dimension or slowdimension for broad-area emitters. Individual beams 104 are illustratedin the figures by a dash or single line, where the length or longerdimension of the beam represents the array dimension or slow divergingdimension for broad-area emitters and the height or shorter dimensionrepresents the fast diverging dimension. (See also the left side of FIG.8). In this related art, a diode bar 102 having four emitters isillustrated. The emitters are aligned in a manner such that the slowdimension ends of each emitted beam 104 are aligned to one another sideby side along a single row—sometimes referred to as an array. However,it is contemplated that any lasing elements may be used and inparticular laser elements with broad gain bandwidth. Typically acollimation lens 106 is used to collimate each beam along the fastdiverging dimension. In some cases the collimation optics can becomposed of separate fast axis collimation lenses and slow axiscollimation lenses. Typically, transform optic 108 is used to combineeach beam along the WBC dimension (or “WBC direction”) 110 as shown bythe input front view 112. Transform optic 108 may be a cylindrical orspherical lens or mirror. The transform optic 108 then overlaps thecombined beam onto a dispersive element 114 (here shown as a reflectingdiffraction grating). The first-order diffracted beams are incident ontoa partially reflecting mirror. The laser resonator is formed between theback facet of the laser elements and the partially reflecting mirror. Assuch, the combined beam is then transmitted as a single output profileonto an output coupler 116. This output coupler then transmits thecombined beams 120, as shown by the output front view 118. It iscontemplated creating a system devoid of an output coupler. Forinstance, a one-dimensional or two-dimensional system with wavelengthstabilized laser elements and each having a unique wavelength may beaccomplished in a few ways. One system or method uses laser elementswith feedback from an external wavelength chirped Volume Bragg gratingalong the beam combining dimension. Another uses internal distributedfeedback (DFB) laser elements or internal distributed Bragg reflector(DBR) laser elements. In these systems, the single output profiletransmitted from the dispersive element would have the same profile as118. The output coupler 116 may be a partially reflective mirror orsurface or optical coating and act as a common front facet for all thelaser elements in diode array 102. A portion of the emitted beams isreflected back into the optical gain and/or lasing portion of diodearray 102 in this external cavity system 100 a. An external cavity is alasing system where the secondary mirror is displaced at a distance awayfrom the emission aperture or facet (not labeled) of each laser emitter.Generally, in an external cavity additional optical elements are placedbetween the emission aperture or facet and the output coupler orpartially reflective surface.

Similarly, FIG. 1B illustrates a stack of laser diode bars each havingfour emitters where those bars are stacked three high. Like FIG. 1A, theinput front view 112 of FIG. 1B, which in this embodiment is atwo-dimensional array of emitters, is combined to produce the outputfront view 118 or a single column of emitters 120. The emitted beams inexternal cavity 100 b were combined along the array dimension. Heretransform optic 108 is a cylindrical lens used to combine the beamsalong the array. However, a combination of optical elements or opticalsystem may be used as such that the optical elements arrange for all thebeams to overlap onto the dispersive element and ensure all the beamsalong the non-beam-combining dimension are propagating normal to theoutput coupler. A simple example of such an optical system would be asingle cylindrical lens with the appropriate focal length along thebeam-combining dimension and two cylindrical lenses that form an afocaltelescope along the non-beam-combining dimension wherein the opticalsystem projects images onto the partially reflecting mirrors. Manyvariations of this optical system can be designed to accomplish the samefunctions.

The array dimension FIG. 1B is also the same axis as the slow dimensionof each emitted beam in the case of multimode diode laser emitters.Thus, this WBC system may also be called slow axis combining, where thecombining dimension is the same dimension of the beams.

By contrast, FIG. 1C illustrates a stack 150 of laser diode arrays 102forming a two-dimensional array of emitters, as shown by 120, whereinstead of combining along the array dimension as in FIGS. 1A and 1B,the WBC dimension now follows along the stack dimension of the emitters.Here, the stacking dimension is also aligned with the fast axisdimension of each of the emitted beams. The input front view 112 is nowcombined to produce the output front view 118 wherein a single column120 of emitters is shown.

There are various drawbacks to all three configurations. One of the maindrawbacks of configuration shown in FIGS. 1A and 1B is that beamcombining is performed along the array dimension. As suchexternal-cavity operation is highly dependent on imperfections of thediode array. If broad-area semiconductor laser emitters are used thespectral utilization in the WBC system is not as efficient as if beamcombining is performed along the fast axis dimension. One of the maindrawbacks of configurations shown in FIG. 1C is that external beamshaping for beam symmetrization is required for efficient coupling intoa fiber. The beam symmetrization optics needed for a high power systemhaving a large number of emitters may be complex and non-trivial.Another disadvantage of configuration 1C is that the output beam qualityis limited to that of a single laser bar. Typical semiconductor or diodelaser bars have 19 to 49 emitters per bar with nearlydiffraction-limited beam quality in one dimension and beam quality thatis several hundreds of times diffraction-limited along the arraydimension. After beam symmetrization the output beam 120 can be coupledinto at best a 100 μm/0.22 Numerical Aperture (NA) fiber. To obtainhigher beam quality a small number of emitter bars is needed. Forexample to couple into 50 μm/0.22 NA fiber a five-emitter output beam isneeded. In many industrial laser applications a higher brightness laserbeam is required. For example, in some applications a two-emitter outputbeam is needed instead of 19 or 49. The two-emitter output beam can becoupled to a smaller core diameter fiber with much more engineeringtolerance and margin. This additional margin in core diameter and NA iscritical for reliable operation at high power (kW-class) power levels.While it is possible to procure five-emitter or two-emitter bars thecost and complexity is generally much higher as compared to standard 19or 49 emitter bars because of the significantly reduced power per bar.In this disclosure, we disclose methods to remove all of the aboveshortcomings.

The previous illustrations, FIGS. 1A-1C, showed pre-arranged or fixedposition arrays and stacks of laser emitters. Generally, arrays orstacks are arranged mechanically or optically to produce a particularone-dimensional or two-dimensional profile. Thus, fixed-position is usedto describe a preset condition of laser elements where the laserelements are mechanically fixed with respect to each other as in thecase of semiconductor or diode laser bars having multiple emitters orfiber lasers mechanically spaced apart in V-grooves, as well as otherlaser emitters that come packaged with the emitters in a fixed position.

Alternatively, fixed position may refer to the secured placement of alaser emitter in a WBC system where the laser emitter is immobile.Pre-arranged refers to an optical array or profile that is used as theinput profile of a WBC system. Often times the pre-arranged position isa result of emitters configured in a mechanically fixed position.Pre-arranged and fixed position may also be used interchangeably.Examples of fixed-position or pre-arranged optical systems are shown inFIGS. 5A-C.

FIGS. 5A-5C refer to prior art illustrated examples of opticallyarranged one and two-dimensional arrays. FIG. 5A illustrates anoptically arranged stack of individual optical elements 510. Mirrors 520are used to arrange the optical beams from optical elements 530, eachoptical element 530 having a near field image 540, to produce an image550 (which includes optical beams from each optical element 530)corresponding to a stack 560 (in the horizontal dimension) of theindividual optical elements 510. Although the optical elements 500 maynot be arranged in a stack, the mirrors 520 arrange the optical beamssuch that the image 550 appears to correspond to the stack 560 ofoptical elements 510. Similarly, in FIG. 5B, the mirrors 520 can be usedto arrange optical beams from diode bars or arrays 570 to create animage 550 corresponding to a stack 560 of diode bars or arrays 575. Inthis example, each diode bar or array 570 has a near field image 540that includes optical beams 545 from each individual element in the baror array. Similarly, the minors 520 may also be used to opticallyarrange laser stacks 580 into an apparent larger overall stack 560 ofindividual stacks 585 corresponding to image 550, as shown in FIG. 5C.

Nomenclature, used in prior art to define the term “array dimension,”referred to one or more laser elements placed side by side where thearray dimension is also along the slow axis. One reason for thisnomenclature is diode bars with multiple emitters are often arranged inthis manner where each emitter is aligned side by side such that eachbeam's slow dimension is along a row or array. For purposes of thisapplication, an array or row refers to individual emitters or beamsarranged across a single dimension. The individual slow or fastdimension of the emitters of the array may also be aligned along thearray dimension, but this alignment is not to be assumed. This isimportant because some embodiments described herein individually rotatethe slow dimension of each beam aligned along an array or row.Additionally, the slow axis of a beam refers to the wider dimension ofthe beam and is typically also the slowest diverging dimension, whilethe fast axis refers to the narrower dimension of the beam and istypically the fastest diverging dimension. The slow axis may also referto single mode beams.

Additionally, some prior art defines the term “stack or stackingdimension” referred to as two or more arrays stacked together, where thebeams' fast dimension is the same as the stacking dimension. Thesestacks were pre-arranged mechanically or optically. However, forpurposes of this application a stack refers to a column of beams orlaser elements and may or may not be along the fast dimension.Particularly, as discussed above, individual beams or elements may berotated within a stack or column.

In some embodiments it is useful to note that the array dimension andthe slow dimension of each emitted beam are initially oriented acrossthe same axis; however, those dimensions, as described in thisapplication, may become oriented at an offset angle with respect to eachother. In other embodiments, the array dimension and only a portion ofthe emitters arranged along the array or perfectly aligned the same axisat a certain position in a WBC system. For example, the array dimensionof a diode bar may have emitters arranged along the array dimension, butbecause of smile (often a deformation or bowing of the bar) individualemitters' slow emitting dimension is slightly skewed or offset from thearray dimension.

Laser sources based on common “commercial, off-the-shelf” or COTS highpower laser diode arrays and stacks are based on broad-areasemiconductor or diode laser elements. Typically, the beam quality ofthese elements is diffraction-limited along the fast axis and many timesdiffraction-limited along the slow axis of the laser elements. It is tobe appreciated that although the following discussion may referprimarily to single emitter laser diodes, diode laser bars and diodelaser stacks, embodiments of the invention are not limited tosemiconductor or laser diodes and may be used with many different typesof laser and amplifier emitters, including fiber lasers and amplifiers,individually packaged diode lasers, other types of semiconductor lasersincluding quantum cascade lasers (QCLs), tapered lasers, ridge waveguide(RWG) lasers, distributed feedback (DFB) lasers, distributed Braggreflector (DBR) lasers, grating coupled surface emitting laser, verticalcavity surface emitting laser (VCSEL), and other types of lasers andamplifiers.

All of the embodiments described herein can be applied to WBC of diodelaser single emitters, bars, and stacks, and arrays of such emitters. Inthose embodiments employing stacking of diode laser elements, mechanicalstacking or optical stacking approaches can be employed. In addition,where an HR coating is indicated at the facet of a diode laser element,the HR coating can be replaced by an AR coating, provided that externalcavity optical components, including but not limited to a collimatingoptic and bulk HR mirror are used in combination with the AR coating.This approach is used, for example, with WBC of diode amplifierelements. Slow axis is also defined as the worse beam quality directionof the laser emission. The slow axis typically corresponds to thedirection parallel to the semiconductor chip at the plane of theemission aperture of the diode laser element. Fast axis is defined asthe better beam quality direction of the laser emission. Fast axistypically corresponds to the direction perpendicular to thesemiconductor chip at the plane of the emission aperture of the diodelaser element.

An example of a single semiconductor chip emitter 1000 is shown in FIG.10. The aperture 1050 is also indicative of the initial beam profile.Here, the height 1010 at 1050 is measured along the stack dimension.Width 1020 at 1050 is measured along the array dimension. Height 1010 isthe shorter dimension at 1050 than width 1020. However, height 1010expands faster or diverges to beam profile 1052, which is placed at adistance away from the initial aperture 1050. Thus, the fast axis isalong the stack dimension. Width 1020 which expands or diverges at aslower rate as indicated by width 1040 being a smaller dimension thanheight 1030. Thus, the slow axis of the beam profile is along the arraydimension. Though not shown, multiple single emitters such as 1000 maybe arranged in a bar side by side along the array dimension.

Drawbacks for combining beams primarily along their slow axis dimensionmay include: reduced power and brightness due to lasing inefficienciescaused by pointing errors, smile and other misalignments. As illustratedin FIG. 2, a laser diode array with smile, one often caused by the diodearray being bowed in the middle sometimes caused by the diode laser barmounting process, is one where the individual emitters along the arrayform a typical curvature representative of that of a smile. Pointingerrors are individual emitters along the diode bar emitting beams at anangle other than normal from the emission point. Pointing error may berelated to smile, for example, the effect of variable pointing along thebar direction of a diode laser bar with smile when the bar is lensed bya horizontal fast axis collimating lens. These errors cause feedbackfrom the external cavity, which consists of the transform lens, grating,and output coupler, not to couple back to the diode laser elements. Somenegative effects of this miscoupling are that the WBC laser breakswavelength lock and the diode laser or related packaging may be damagedfrom miscoupled or misaligned feedback not re-entering the optical gainmedium. For instance the feedback may hit some epoxy or solder incontact or in close proximity to a diode bar and cause the diode bar tofail catastrophically.

Row 1 of FIG. 2 shows a single laser diode bar 202 without any errors.The embodiments illustrated are exemplary of a diode bar mounted on aheat sink and collimated by a fast-axis collimation optic 206. Column Ashows a perspective or 3-D view of the trajectory of the output beams211 going through the collimation optic 206. Column D shows a side viewof the trajectory of the emitted beams 211 passing through thecollimation optic 206. Column B shows the front view of the laser facetwith each individual laser element 213 with respect to the collimationoptic 206. As illustrated in row 1, the laser elements 213 are perfectlystraight. Additionally, the collimation optic 206 is centered withrespect to all the laser elements 213. Column C shows the expectedoutput beam from a system with this kind of input. Row 2 illustrates adiode laser array with pointing error. Shown by column B of row 2 thelaser elements and collimation optic are slightly offset from eachother. The result, as illustrated, is the emitted beams having anundesired trajectory that may result in reduced lasing efficiency for anexternal cavity. Additionally, the output profile may be offset torender the system ineffective or cause additional modifications. Row 3shows an array with packaging error. The laser elements no longer sit ona straight line, and there is curvature of the bar. This is sometimesreferred to as “smile.” As shown on row 3, smile may introduce even moretrajectory problems as there is no uniform path or direction common tothe system. Column D of row 3 further illustrates beams 211 exiting atvarious angles. Row 4 illustrates a collimation lens unaligned with thelaser elements in a twisted or angled manner. The result is probably theworst of all as the output beams generally have the most collimation ortwisting errors. In most systems, the actual error in diode arrays andstacks is a combination of the errors in rows 2, 3, and 4. In bothmethods 2 and 3, using VBGs and diffraction gratings, laser elementswith imperfections result in output beams no longer pointing parallel tothe optical axis. These off optical axis beams result in each of thelaser elements lasing at different wavelengths. The plurality ofdifferent wavelengths increases the output spectrum of the system tobecome broad as mentioned above.

One of the advantages of performing WBC along the stacking dimension(here also primarily the fast dimension) of a stack of diode laser barsis that it compensates for smile as shown in FIG. 2. Pointing and otheralignment errors are not compensated by performing WBC along the arraydimension (also primarily slow dimension). A diode bar array may have arange of emitters typically from 19 to 49 or more. As noted, diode bararrays are typically formed such that the array dimension is where eachemitter's slow dimension is aligned side by side with the otheremitters. As a result, when using WBC along the array dimension, whethera diode bar array has 19 or 49 emitters (or any other number ofemitters), the result is that of a single emitter. By contrast, whenperforming WBC along the orthogonal or fast dimension of the same singlediode bar array, the result is each emitted beam increases in spectralbrightness, or narrowed spectral bandwidth, but there is not a reductionin the number of beams (equivalently, there is not an increase inspatial brightness).

This point is illustrated in FIG. 8. On the left of FIG. 8 is shown afront view of an array of emitters 1 and 2 where WBC along the slowdimension is being performed. Along the right side using the same arrays1 and 2, WBC along the fast dimension is being performed. When comparingarray 1, WBC along the slow dimension reduces the output profile to thatof a single beam, while WBC along the fast dimension narrows thespectral bandwidth, as shown along the right side array 1, but does notreduce the output profile size to that of a single beam.

Using COTS diode bars and stacks the output beam from beam combiningalong the stack dimension is usually highly asymmetric. Symmetrization,or reducing the beam profile ratio closer to equaling one, of the beamprofile is important when trying to couple the resultant output beamprofile into an optical fiber. Many of the applications of combining aplurality of laser emitters require fiber coupling at some point in anexpanded system. Thus, having greater control over the output profile isanother advantage of the application.

Further analyzing array 2 in FIG. 8 shows the limitation of the numberof emitters per laser diode array that is practical for performing WBCalong the fast dimension if very high brightness symmetrization of theoutput profile is desired. As discussed above, typically the emitters ina laser diode bar are aligned side by side along their slow dimension.Each additional laser element in a diode bar is going to increase theasymmetry in the output beam profile. When performing WBC along the fastdimension, even if a number of laser diode bars are stacked on eachother, the resultant output profile will still be that of a single laserdiode bar. For example if one uses a COTS 19-emitter diode laser bar,the best that one can expect is to couple the output into a 100 μm/0.22NA fiber. Thus, to couple into a smaller core fiber, a lower number ofemitters per bar is required. One could simply fix the number ofemitters in the laser diode array to 5 emitters in order to help withthe symmetrization ratio; however, fewer emitters per laser diode bararray generally results in an increase of cost per bar or cost per Wattof output power. For instance, the cost of a diode bar having 5 emittersmay be roughly the same as that of a diode bar having 49 emitters.However, the 49 emitter bar may have a total power output of up to anorder-of-magnitude greater than that of the 5 emitter bar. Thus, itwould be advantageous for a WBC system to be able to achieve very highbrightness output beams using COTS diode bars and stacks with largernumber of emitters. An additional advantage of bars with larger numberof emitters is the ability to de-rate the power per emitter to achieve acertain power level per bar for a given fiber-coupled power level,thereby increasing the diode laser bar lifetime or bar reliability.

One embodiment that addresses this issue is illustrated in FIG. 3A,which shows a schematic of WBC system 300 a with an optical rotator 305placed after collimation lenses 306 and before the transform optic 308.It should be noted the transform optic 308 may include or consistessentially of a number of lenses or mirrors or other opticalcomponents. The optical rotator 305 individually rotates the fast andslow dimension of each emitted beam shown in the input front view 312 toproduce the re-oriented front view 307. It should be noted that theoptical rotators can selectively rotate each beam individuallyirrespective of the other beams or can rotate all the beams through thesame angle simultaneously. It should also be noted that a cluster of twoor more beams can be rotated simultaneously. The resulting output afterWBC is performed along the array dimension is shown in output front view318 as a single emitter. Dispersive element 314 is shown as a reflectiondiffraction grating, but may also be a dispersive prism, a grism(prism+grating), transmission grating, and Echelle grating. Thisparticular embodiment illustrated shows only four laser emitters;however, as discussed above this system could take advantage of a laserdiode array that included many more elements, e.g., 49. This particularembodiment illustrated shows a single bar at a particular wavelengthband (example at 976 nm) but in actual practice it may be composed ofmultiple bars, all at the same particular wavelength band, arrangedside-by-side. Furthermore, multiple wavelength bands (example 976 nm,915 nm, and 808 nm), with each band composing of multiple bars, may becombined in a single cavity. Because WBC was performed across the fastdimension of each beam it was easier to design a system with a higherbrightness (higher efficiency due to insensitivity due to barimperfections); additionally, narrower bandwidth and higher power outputare all achieved. As previously discussed it should be noted that insome embodiments WBC system 300 a may not include output coupler 316and/or collimation lens(es) 306. Furthermore, pointing errors and smileerrors are compensated for by combining along the stack dimension (inthis embodiment this is also the fast dimension). FIG. 3B, shows animplementation similar to 3A except that a stack 350 of laser arrays 302forms a 2-D input profile 312. Cavity 300 b similarly consists ofcollimation lens(es) 306, optical rotator 305, transform optic 308,dispersive element 314 (here a diffraction grating), and an outputcoupler 316 with a partially reflecting surface. Each of the beams isindividually rotated by optical rotator 305 to form an after rotatorprofile 307. The WBC dimension is along the array dimension, but withthe rotation each of the beams will be combined across their fast axis.Fast axis WBC produces outputs with very narrow line widths and highspectral brightness. These are usually ideal for industrial applicationssuch as welding. After transform optic 308 overlaps the rotated beamsonto dispersive element 314 a single output profile is produced andpartially reflected back through the cavity into the laser elements. Theoutput profile 318 is now comprised of a line of three (3) beams that isquite asymmetric.

FIG. 3C shows the same implementation when applied to 2-D laserelements. The system consists of 2-D laser elements 302, optical rotator305, transform optical system (308 and 309 a-b) a dispersive element314, and a partially reflecting mirror 316. FIG. 3C illustrates a stack350 of laser diode bars 302 with each bar having an optical rotator 305.Each of the diode bars 302 (three total) as shown in external cavity 300c includes four emitters. After input front view 312 is reoriented byoptical rotator 305, reoriented front view 307 shows the slow dimensionof each beam aligned along the stack dimension. WBC is performed alongthe dimension, which is now the slow axis of each beam and the outputfront view 318 now comprises a single column of beams with each beam'sslow dimension oriented along the stack dimension. Optic 309 a and 309 bprovide a cylindrical telescope to image along the array dimension. Thefunction of the three cylindrical lenses is two-fold. The middlecylindrical lens is the transform lens and its main function is tooverlap all the beams onto the dispersive element. The two othercylindrical lenses 309 a and 309 b form an afocal cylindrical telescopealong the non-beam combining dimension. Its main function is to makesure all laser elements along the non-beam combining dimension arepropagating normal to the partially reflecting mirror. As such theimplementation as shown in FIG. 3C has the same advantages as the oneshown in FIG. 1C. However, unlike the implementation as shown in FIG. 1Cthe output beam is not the same as the input beam. The number ofemitters in the output beam 318 in FIG. 3C is the same as the number ofbars in the stack. For example, if the 2-D laser source consists of athree-bar stack with each bar composed of 49 emitters, then the outputbeam in FIG. 1C is a single bar with 49 emitters. However, in FIG. 3Cthe output beam is a single bar with only three emitters. Thus, theoutput beam quality or brightness is more than one order of magnitudehigher. This brightness improvement is very significant forfiber-coupling. For higher power and brightness scaling multiple stackscan be arranged side-by-side.

To illustrate this configuration further, for example, assume WBC is tobe performed of a three-bar stack, with each bar comprising of 19emitters. So far, there are three options. First, wavelength beamcombining can be performed along the array dimension to generate threebeams as shown in FIG. 1B. Second, wavelength beam combining can beperformed along the stack dimension to generate 19 beams a shown FIG.1C. Third, wavelength beam combining can be performed along the arraydimension using beam rotator to generate 19 beams as shown FIG. 3C.There are various trade-offs for all three configuration. The first casegives the highest spatial brightness but the lowest spectral brightness.The second case gives the lowest spatial brightness with moderatespectral brightness and beam symmetrization is not required to coupleinto a fiber. The third case gives the lowest spatial brightness but thehighest spectral brightness and beam symmetrization is required tocouple into an optical fiber. In some applications this more desirable.

To illustrate the reduction in asymmetry FIG. 3D has been drawn showingthe final output profile 319 a where the system of 300 b did not have anoptical rotator and output profile 319 b where the system includes anoptical rotator. Though these figures are not drawn to scale, theyillustrate an advantage achieved by utilizing an optical rotator, in asystem with this configuration where WBC is performed across the slowdimension of each beam. The shorter and wider 319 b is more suitable forfiber coupling than the taller and slimmer 319 a.

Examples of various optical rotators are shown in FIG. 4A-4C. FIG. 4Aillustrates an array of cylindrical lenses (419 a and 419 b) that causeinput beam 411 a to be rotated to a new orientation at 411 b. FIG. 4Bsimilarly shows input 411 a coming into the prism at an angle, whichresults in a new orientation or rotation beam 411 b. FIG. 4C illustratesan embodiment using a set of step mirrors 417 to cause input 411 a torotate at an 80-90 degree angle with the other input beams resulting ina new alignment of the beams 411 b where they are side by side alongtheir respective fast axis. These devices and others may cause rotationthrough both non-polarization sensitive as well as polarizationsensitive means. Many of these devices become more effective if theincoming beams are collimated in at least the fast dimension. It is alsounderstood that the optical rotators can selectively rotate the beams byvarious amounts, including less than 90 degrees, 90 degrees, and greaterthan 90 degrees.

The optical rotators in the previous embodiments may selectively rotateindividual, rows or columns, and groups of beams. In some embodiments aset angle of rotation, such as a range of 80-90 degrees is applied tothe entire profile or subset of the profile. In other instances, varyingangles of rotation are applied uniquely to each beam, row, column orsubset of the profile (see FIGS. 9A-B). For instance, one beam may berotated by 45 degrees in a clockwise direction while an adjacent beam isrotated 45 degrees in a counterclockwise direction. It is alsocontemplated one beam is rotated 10 degrees and another is rotated 70degrees. The flexibility the system provides may be applied to a varietyof input profiles, which in turn helps determine how the output profileis to be formed.

Performing WBC along an intermediate angle between the slow and fastdimension of the emitted beams is also well within the scope of theinvention (See for example 6 on FIG. 9B). Some laser elements asdescribed herein, produce electromagnetic radiation and include anoptical gain medium. When the radiation or beams exit the optical gainportion they generally are collimated along the slow and/or fastdimension through a series of micro lenses. From this point, theembodiments already described in this section included an opticalrotator that selectively rotated each beam prior to the beams beingoverlapped by a transform lens along either the slow or the fastdimension of each beam onto a dispersive element. The output coupler mayor may not be coated to partially reflect the beams back into the systemto the laser element where the returned beams assist in generating moreexternal cavity feedback in the optical gain element portion until theyare reflected off a fully reflective mirror in the back portion of thelaser element. The location of the optical elements listed above andothers not listed with respect to the second partially reflectivesurface helps determine whether the optical elements are within anexternal cavity system or outside of the lasing cavity. In someembodiments, not shown, the second partially reflective mirror residesat the end of the optical gain elements and prior to the collimating orrotating optics.

Another method for manipulating beams and configurations to takeadvantage of the various WBC methods includes using a spatialrepositioning element. This spatial repositioning element may be placedin an external cavity at a similar location as that of an opticalrotator. For example, FIG. 6 shows a spatial repositioning element 603placed in the external cavity WBC system 600 after the collimatinglenses 606 and before the transform optic(s) 608. The purpose of aspatial repositioning element is to reconfigure an array of elementsinto a new configuration. FIG. 6 shows a three-bar stack with N elementsreconfigured to a six-bar stack with N/2 elements. Spatial repositioningis particularly useful in embodiments such as 600, where stack 650 is amechanical stack or one where diode bar arrays 602 and their outputbeams were placed on top of each other either mechanically or optically.With this kind of configuration the laser elements have a fixed-positionto one another. Using a spatial repositioning element can form a newconfiguration that is more ideal for WBC along the fast dimension. Thenew configuration makes the output profile more suitable for fibercoupling.

For example, FIG. 7 illustrates an embodiment wherein a two-dimensionalarray of emitters 712 is reconfigured during a spatial repositioningstep 703 by a spatial repositioning optical element such as an array ofperiscope mirrors. The reconfigured array shown by reconfigured frontview 707 is now ready for a WBC step 710 to be performed across the WBCdimension, which here is the fast dimension of each element. Theoriginal two-dimensional profile in this example embodiment is an arrayof 12 emitters tall and 5 emitters wide. After the array is transmittedor reflected by the spatial repositioning element a new array of 4elements tall and 15 elements wide is produced. In both arrays theemitters are arranged such that the slow dimension of each is verticalwhile the fast dimension is horizontal. WBC is performed along the fastdimension which collapses the 15 columns of emitters in the second arrayinto 1 column that is 4 emitters tall. This output is already moresymmetrical than if WBC had been performed on the original array, whichwould have resulted in a single column 15 emitters tall. As shown, thisnew output may be further symmetrized by an individually rotating step705 rotating each emitter by 90 degrees. In turn, a post-WBC front view721 is produced being the width of a single beam along the slowdimension and stacked 4 elements high, which is more suitable forcoupling into a fiber.

One way of reconfiguring the elements in a one-dimensional ortwo-dimensional profile is to make ‘cuts’ or break the profile intosections and realign each section accordingly. For example, in FIG. 7two cuts 715 were made in 713. Each section was placed side by side toform 707. These optical cuts can be appreciated if we note the elementsof 713 had a pre-arranged or fixed-position relationship. It is alsowell within the scope to imagine any number of cuts being made toreposition the initial input beam profile. Each of these sections may inaddition to being placed side by side, be on top and even randomized ifso desired.

Spatial repositioning elements may be comprised of a variety of opticalelements including periscope optics that are both polarized andnon-polarized as well as other repositioning optics. Step mirrors asshown in FIG. 4 a may also be reconfigured to become a spatialrepositioning element.

Additional embodiments of the invention are illustrated in FIGS. 9A-9B.The system shown in 1 of FIG. 9A shows a single array of four beamsaligned side to side along the slow dimension. An optical rotatorindividually rotates each beam. The beams are then combined along thefast dimension and are reduced to a single beam by WBC. In thisarrangement it is important to note that the 4 beams could easily be 49or more beams. It may also be noted that if some of the emitters arephysically detached from the other emitters, the individual emitter maybe mechanically rotated to be configured in an ideal profile. Amechanical rotator may be comprised of a variety of elements includingfriction sliders, locking bearings, tubes, and other mechanismsconfigured to rotate the laser element. Once a desired position isachieved the laser elements may then be fixed into place. It is alsoconceived that an automated rotating system that can adjust the beamprofile depending on the desired profile may be implemented. Thisautomated system may either mechanically reposition a laser or opticalelement or a new optical element may be inserted in and out of thesystem to change the output profile as desired.

System 2 shown in FIG. 9A, shows a two-dimensional array having threestacked arrays with four beams each aligned along the slow dimension.(Similar to FIG. 3C) As this stacked array passes through an opticalrotator and WBC along the fast dimension a single column of three beamstall aligned top to bottom along the slow dimension is created. Again itis appreciated that if the three stacked arrays shown in this system had50 elements, the same output profile would be created, albeit one thatis brighter and has a higher output power.

System 3 in FIG. 9B, shows a diamond pattern of four beams wherein thebeams are all substantially parallel to one another. This pattern mayalso be indicative of a random pattern. The beams are rotated andcombined along the fast dimension, which results in a column of threebeams aligned along the slow dimension from top to bottom. Missingelements of diode laser bars and stacks due to emitter failure or otherreasons is an example of System 3. System 4 illustrates a system wherethe beams are not aligned, but that one beam is rotated to be alignedwith a second beam such that both beams are combined along the fastdimension forming a single beam. System 4 demonstrates a number ofpossibilities that expands WBC methods beyond using laser diode arrays.For instance, the input beams in System 4 may be from carbon dioxide(CO₂) lasers, semiconductor or diode lasers, diode pumped fiber lasers,lamp-pumped or diode-pumped Nd:YAG lasers, Disk Lasers, and so forth.The ability to mix and match the type of lasers and wavelengths oflasers to be combined is another advantage of embodiments of the presentinvention.

System 5 illustrates a system where the beams are not rotated to befully aligned with WBC dimension. The result is a hybrid output thatmaintains many of the advantages of WBC along the fast dimension. Inseveral embodiments the beams are rotated a full 90 degrees to becomealigned with WBC dimension, which has often been the same direction ordimension as the fast dimension. However, System 5 and again System 6show that optical rotation of the beams as a whole (System 6) orindividually (System 5) may be such that the fast dimension of one ormore beams is at an angle theta or offset by a number of degrees withrespect to the WBC dimension. A full 90 degree offset would align theWBC dimension with the slow dimension while a 45 degree offset wouldorient the WBC dimension at an angle halfway between the slow and fastdimension of a beam as these dimension are orthogonal to each other. Inone embodiment, the WBC dimension has an angle theta at approximately 3degrees off the fast dimension of a beam.

As described above, the output beam of a WBC laser system mayadvantageously be coupled into an optical fiber for use in a variety ofdifferent applications. Embodiments of the present invention utilize thefiber itself (or a portion thereof or an interface therein or thereto)as the partially reflective output coupler, which in part establishesthe WBC external lasing cavity, in order to reduce component count andsystem complexity. FIGS. 11A and 11B depict an optical fiber 1100 thatmay both (1) be utilized as a partially reflective output coupler in aWBC laser system and (2) deliver the multi-wavelength output beam. Asshown, laser light 1105 is focused into fiber 1100 via an optional endcap 1110, which typically includes or consists essentially of glass. Theend cap 1110 may have an anti-reflection coating 1115 on one or moresurfaces thereof. End cap 1110 (and corresponding end cap 1120 at theopposite end of fiber 1100, which may also have an anti-reflectioncoating 1115 thereon) reduce the intensity (i.e., light power per unitarea) at the surfaces of the fiber to minimize or substantiallyeliminate damage to any anti-reflection coatings and reduce the risk ofsurface contamination that might cause catastrophic failure of the fiberand/or laser system. The laser light 1105 is coupled into core 1125and/or cladding 1130 of the fiber via end cap 1110, and one or morereflective structures and/or interfaces within the fiber 1100 reflect afirst portion 1135 of the light 1105 back toward the dispersive element(e.g., a diffraction grating) of the WBC system while transmitting asecond portion 1140 of the light 1105 as a multi-wavelength output beam.The end caps 1110, 1120 may include, consist essentially of, or consistof the same material as a material of at least part of the optical fiber(e.g., the core 1125 and/or the cladding 1130).

As shown in FIGS. 11A and 11B, the partially reflective structure withinthe fiber 1100 may include or consist essentially of one or more fiberBragg gratings 1145. As known to those of skill in the art, the fiberBragg grating 1145 includes or consists essentially of a periodicvariation of the refractive index of a portion of the fiber 1100 (e.g.,within the core 1125). The period variation may be, e.g., on the orderof one-half of the wavelength (or one of the wavelengths) of light 1105,and the grating 1145 thus induces Fresnel reflection. The wavelengthdependence and/or the magnitude of the reflection may be selected by theparticular grating pattern and the refractive-index variation therein.As shown in FIG. 11B, multiple fiber Bragg gratings 1145 may be disposedwithin the fiber 1100, and each grating 1145 may have a differentrefractive-index variation and/or wavelength selectivity.

FIG. 11C depicts the fiber 1100 within an exemplary WBC laser system1150. In WBC system 1150, beams are emitted by a one-dimensional ortwo-dimensional array of beam emitters 102. The beams may be manipulatedby one or more (e.g., one for each beam, or one shared by all beams)optics 1155. Optics 1155 collimate, rotate, and/or otherwise manipulatethe beams emitted by beam emitters 102, and optics 1155 may include orconsist essentially of, for example, an optical rotator (as detailedabove and/or as described in U.S. patent application Ser. No.14/734,480, filed Jun. 9, 2015, the entire disclosure of which isincorporated by reference herein) or a micro-optics arrangement (e.g., aslow-axis collimation lens, an optical twister, and/or a fast-axiscollimation lens, as described in U.S. patent application Ser. No.14/667,094, filed Mar. 24, 2015, the entire disclosure of which isincorporated by reference herein).

The WBC system 1150 also includes focusing optics 108 (e.g., one or morecylindrical lenses and/or mirrors, and/or one or more spherical lensesand/or mirrors) that combine the beams emitted by the emitters 102 toform a combined beam that propagates toward a dispersive element 114.The dispersive element 114 (e.g., a diffraction grating, a dispersiveprism, a grism (prism/grating), a transmission grating, or an Echellegrating) may be disposed approximately at the focal length of thefocusing optics 108. As described herein, the dispersive element 114receives the combined beam and transmits the beam as a multi-wavelengthbeam having a high brightness. The multi-wavelength beam is focused byoptics 1160 (e.g., one or more spherical and/or cylindrical lenses) andtransmitted to the optical fiber 1100, which, via fiber Bragg grating1145, transmits a portion of the multi-wavelength beam and reflects asecond portion of the multi-wavelength beam back toward the dispersiveelement 114 and thence to the emitters 102, forming an external lasingcavity. As shown in FIG. 11C, the grating 1145 may include or consist ofregions, having a period 1160, of varying refractive index. For example,the grating 1145 may include regions having a refractive index n₃different from (e.g., larger than) the refractive index n₂ of the coreof fiber 1100. (Fiber 1100 is also depicted as having a cladding 1130having a refractive index n₁ and being surrounded by air having arefractive index n₀.)

FIGS. 12A and 12B depict an optical fiber 1200 that may, in accordancewith various embodiments of the present invention, both (1) be utilizedas a partially reflective output coupler in a WBC laser system and (2)deliver the multi-wavelength output beam. As shown, laser light 1105 isfocused into fiber 1200 and partially reflected by a partiallyreflective coating (e.g., a coating having a reflectivity to one or morewavelengths of the light 1105 ranging from about 1% to about 10%, oreven more) or surface. As detailed herein, the reflected portion 1135 oflight 1105 establishes the WBC external cavity in tandem with thesource(s) of the light 1105, and the transmitted multi-wavelengthportion 1140 of the light propagates through the fiber 1200 and may beutilized in any of a variety of applications.

In various embodiments of the invention, the partially reflectivecoating and/or surface may include or consist essentially of a partiallyreflective coating on a surface of the fiber and/or on the end cap 1110(e.g., at the interface between the end cap and the fiber), or a coatingor surface between two segments of fiber (or, equivalently, two fibers).In the configuration of FIG. 12A, a partially reflective coating 1210 isdisposed between the fiber and the end cap 1110. For example, thecoating 1210 may be disposed on the surface of the fiber core 1125 andfiber cladding 1130 facing the end cap 1110 and/or on the surface of theend cap 1110 itself. The coating 1210 may include or consist essentiallyof, e.g., one or more metallic layers and/or one or more dielectriclayers. As in fiber 1100 described above, the coating 1210 may partiallyreflect the light 1105 via Fresnel reflection.

In the configuration of FIG. 12B, the coating 1210 is disposed betweentwo fibers or fiber segments (which may be, for example, splicedtogether). As shown, this configuration may also include an optionaladapter 1220 optically downstream of the coating 1210 in order to moreefficiently capture and in-couple the transmitted multi-wavelength beam1140 into the second fiber or fiber portion. For example, the adapter1220 may include or consist essentially of a fiber taper (i.e., acoherent fiber optic plate that transmits either a magnified or reducedimage from its input surface to its output surface) or mode fieldadapter (i.e., a splicing device to join together, within minimal lightloss, fibers, particularly fibers with different mode fields (e.g.,different core diameters and/or numerical apertures)). In variousembodiments, the adapter 1220 may include or consist essentially of aportion of fiber core 1125 that is larger (i.e., has a larger diameter)than the core 1125 downstream of the adapter 1220.

FIG. 12C depicts the fiber 1200 within an exemplary WBC laser system1250. In WBC system 1250, beams are emitted by a one-dimensional ortwo-dimensional array of beam emitters 102. As in WBC laser system 1150,the beams may be manipulated by one or more (e.g., one for each beam, orone shared by all beams) optics 1155. Optics 1155 collimate, rotate,and/or otherwise manipulate the beams emitted by beam emitters 102. TheWBC system 1250 also includes focusing optics 108 that combine the beamsemitted by the emitters 102 to form a combined beam that propagatestoward dispersive element 114. The dispersive element 114 (e.g., adiffraction grating, a dispersive prism, a grism (prism/grating), atransmission grating, or an Echelle grating) may be disposedapproximately at the focal length of the focusing optics 108. Asdescribed herein, the dispersive element 114 receives the combined beamand transmits the beam as a multi-wavelength beam having a highbrightness. The multi-wavelength beam is focused by optics 1160 (e.g.,one or more spherical and/or cylindrical lenses) and transmitted to theoptical fiber 1200, which, via coating or surface 1210, transmits aportion of the multi-wavelength beam and reflects a second portion ofthe multi-wavelength beam back toward the dispersive element 114 andthence to the emitters 102, forming an external lasing cavity.

Various embodiments of the present invention incorporate a partiallyreflective output coupler 116 between two fibers (or fiber segments), asdepicted in FIG. 13. FIG. 13 shows a fiber assembly 1300 in which light1105 is focused and in-coupled into a first fiber 1310. The light 1105subsequently exits the fiber 1310 and encounters the output coupler 116,where first portion 1135 is reflected back into fiber 1310 and backtoward the beam emitter(s) (thus establishing the external WBC cavity),and multi-wavelength output portion 1140 enters a second fiber 1320 fordelivery and use in any of a variety of different applications. Asshown, the assembly 1300 may also incorporate end caps 1330 and/orfocusing optics 1340 (e.g., one or more spherical and/or cylindricallenses) on the ends of the fibers 1310, 1320 that face the outputcoupler 116. FIGS. 14A and 14B depict an optical fiber 1400 that may, inaccordance with various embodiments of the present invention, both (1)be utilized as a partially reflective output coupler in a WBC lasersystem and (2) deliver the multi-wavelength output beam. As shown, laserlight 1105 is focused into fiber 1400 and partially reflected by aninterface having different refractive indices on either side. Asdetailed herein, the reflected portion 1135 of light 1105 establishesthe WBC external cavity in tandem with the source(s) of the light 1105,and the transmitted multi-wavelength portion 1140 of the lightpropagates through the fiber 1400 and may be utilized in any of avariety of applications.

In various embodiments of the invention, the partially reflectiveinterface 1410 may lie between the core 1125 and/or cladding 1130 of thefiber and an end cap 1420 that includes, consists essentially of, orconsists of a material different from that of the core 1125 and/orcladding 1130, as shown in FIG. 14A. For example, the core 1125 and/orcladding 1130 may include or consist essentially of glass or fusedsilica, while the end cap 1420 may include or consist essentially ofsapphire. While FIG. 14A depicts the entire end cap 1420 as beingcomposed of a material different from that of the remaining parts of thefiber, in various embodiments only a portion of end cap 1420 (e.g., theportion facing and/or in contact with core 1125 and/or cladding 1130) iscomposed of the different material.

As shown in FIG. 14B, the partially reflective interface 1410 may alsobe established between the material of the end cap 1110 and/or core 1125and/or cladding 1130 (e.g., silica) and a void (i.e., vacuum) or airregion (e.g., bubble) 1430 disposed within the fiber 1400. In anexemplary embodiment in which the end cap 1110 is composed of silica andregion 1430 comprises air, interface 1410 may have approximately 4%reflectivity. Other materials combinations may be utilized to tailor thereflectivity and/or wavelength dependence of partially reflectiveinterface 1410. In various embodiments, the interface 1410 may bedisposed on the exit surface of end cap 1120 (or of the fiber itself, ifend cap 1120 is not present) when that surface lacks an anti-reflectivecoating (e.g., anti-reflective coating 1115) thereon. The resultingglass/air interface may have approximately 4% reflectivity and act asthe partially reflective output coupler, reflecting light portion 1135to establish the external cavity and transmitting multi-wavelengthoutput beam 1140.

In various embodiments of the present invention, the partiallyreflective output coupler 116 is disposed optically downstream of theoptical fiber, which is thereby disposed within the external WBC cavity.FIG. 15 depicts an exemplary WBC laser system 1500 that incorporates anoptical fiber 1510 within the WBC external cavity. As shown, in WBCsystem 1500, beams are emitted by a one-dimensional or two-dimensionalarray of beam emitters 102. As in WBC laser system 1150, the beams maybe manipulated by one or more (e.g., one for each beam, or one shared byall beams) optics 1155. Optics 1155 collimate, rotate, and/or otherwisemanipulate the beams emitted by beam emitters 102. The WBC system 1500also includes focusing optics 108 that combine the beams emitted by theemitters 102 to form a combined beam that propagates toward dispersiveelement 114. The dispersive element 114 (e.g., a diffraction grating, adispersive prism, a grism (prism/grating), a transmission grating, or anEchelle grating) may be disposed approximately at the focal length ofthe focusing optics 108. As described herein, the dispersive element 114receives the combined beam and transmits the beam as a multi-wavelengthbeam having a high brightness. The multi-wavelength beam is focused byoptics 1160 (e.g., one or more spherical and/or cylindrical lenses) andtransmitted to the optical fiber 1510, which may incorporate end caps asdescribed herein. After propagating through the optical fiber 1510, thelight is focused by optics 1520 (e.g., one or more cylindrical and/orspherical lenses) onto partially reflective output coupler 116, whichtransmits a portion of the multi-wavelength beam and reflects a secondportion of the multi-wavelength beam back toward the dispersive element114 and thence to the emitters 102, forming the external lasing cavity.In such embodiments, the output coupler 116 (and, in some embodiments,optics 1520) may be disposed within a processing laser head 1530 that isdiscrete from (and may be detachable from) the other components of lasersystem 1500 and that delivers the multi-wavelength output beam to aworkpiece for applications such as cutting, welding, etc.

WBC laser systems in accordance with various embodiments of the presentinvention (e.g. WBC laser systems 1150, 1250, 1500) have advantages overconventional systems. First, the efficiency of the WBC laser may beincreased due to reduced losses in fiber coupling, as the output coupler(and at least a portion of the fiber) is integrated into the externalcavity. Second, the fiber may serve as a mode filter for the externalcavity, selecting the desirable modes for efficient fiber coupling. Forthis reason the external cavity laser threshold may be reduced forlasers in accordance with embodiments of the invention (e.g., WBC lasersystem 1250). Third, the AR coating at the fiber input end may beomitted, which eliminates the risk of damage to the AR coating at thisinterface. AR coatings may be a failure point for optical components ingeneral, since AR coatings often have a lower damage threshold ascompared with bulk glass. A fourth advantage is a built-in safetymechanism at the input fiber connectorization point. If the fiber isdisconnected from the laser and current is applied to the diode laser,owing to the low-reflectivity facet coating and the presence of theexternal cavity, the diode laser will not lase in the external cavitymode and instead, only weak incoherent emission will typically emergefrom the output port. The WBC laser does not lase in the external cavitywithout the fiber (and any fiber connector(s)) attached. Thus, the lasermay in general not be turned on and used to obtain significant outputwithout having the fiber (and any fiber connector(s)) installed. Thisfeature makes the WBC laser relatively safe to prevent high opticaloutput power in free space when the output fiber is disconnected.

The terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:
 1. A laser system comprising: an array of beamemitters each emitting a beam; focusing optics for focusing the beamstoward a dispersive element; a dispersive element for receiving anddispersing the focused beams, thereby forming a multi-wavelength beam;an optical fiber for receiving the multi-wavelength beam; and disposedwithin the optical fiber, a fiber Bragg grating for receiving themulti-wavelength beam, reflecting a first portion thereof back towardthe dispersive element, and transmitting a second portion thereof as anoutput beam composed of multiple wavelengths, wherein a portion of theoptical fiber is disposed within an external lasing cavity establishedbetween the array of beam emitters and the fiber Bragg grating.
 2. Thelaser system of claim 1, wherein (i) the optical fiber comprises one ormore cores at least partially surrounded by one or more cladding layers,and (ii) at least a portion of the fiber Bragg grating is disposedwithin at least one core of the optical fiber.
 3. The laser system ofclaim 1, wherein the dispersive element comprises a diffraction grating.4. The laser system of claim 1, wherein the focusing optics comprises atleast one of a cylindrical lens or a cylindrical mirror.
 5. The lasersystem of claim 1, further comprising a first end cap attached to theoptical fiber and disposed optically upstream of the fiber Bragggrating.
 6. The laser system of claim 1, further comprising a second endcap attached to the optical fiber and disposed optically downstream ofthe fiber Bragg grating.
 7. A laser system comprising: an array of beamemitters each emitting a beam; focusing optics for focusing the beamstoward a dispersive element; a dispersive element for receiving anddispersing the focused beams, thereby forming a multi-wavelength beam;an optical fiber for receiving the multi-wavelength beam; and disposedwithin or on the optical fiber, a partially reflective interface forreceiving the multi-wavelength beam, reflecting a first portion thereofback toward the dispersive element, and transmitting a second portionthereof as an output beam composed of multiple wavelengths.
 8. The lasersystem of claim 7, further comprising an end cap attached to the opticalfiber, the partially reflective interface being disposed between the endcap and the optical fiber.
 9. The laser system of claim 8, wherein (i)the end cap comprises a first material, (ii) the optical fiber comprisesa second material, and (iii) the first material is different from thesecond material.
 10. The laser system of claim 9, wherein the firstmaterial comprises sapphire and the second material comprises glass. 11.The laser system of claim 8, wherein the partially reflective interfacecomprises a coating applied to a surface of the end cap and/or a surfaceof the optical fiber.
 12. The laser system of claim 7, wherein thepartially reflective interface is disposed between a portion of theoptical fiber and a void disposed within the optical fiber.
 13. Thelaser system of claim 12, wherein the void comprises therewithin air orvacuum.
 14. The laser system of claim 7, wherein (i) the optical fibercomprises first and second optical fiber portions joined together, and(ii) the partially reflective interface is disposed between the firstand second optical fiber portions.
 15. The laser system of claim 14,further comprising a mode field adaptor or a fiber taper disposedoptically downstream of the partially reflective interface.
 16. Thelaser system of claim 14, wherein (i) the first optical fiber portioncomprises a core region optically upstream of the partially reflectiveinterface, (ii) the second optical fiber portion comprises a core regionoptically downstream of the partially reflective interface, and (iii) adiameter of the core region of the second optical fiber portion islarger than a diameter of the core region of the first optical fiberportion.
 17. A laser system comprising: an array of beam emitters eachemitting a beam; focusing optics for focusing the beams toward adispersive element; a dispersive element for receiving and dispersingthe focused beams, thereby forming a multi-wavelength beam; a firstoptical fiber for receiving the multi-wavelength beam; a second opticalfiber disposed optically downstream of the first optical fiber; anddisposed between the first and second optical fibers, a partiallyreflective output coupler for receiving the multi-wavelength beam,reflecting a first portion thereof back toward the dispersive element,and transmitting a second portion thereof as an output beam composed ofmultiple wavelengths.
 18. The laser system of claim 17, furthercomprising a first end cap and/or a first focusing lens disposed betweenthe first optical fiber and the partially reflective output coupler. 19.The laser system of claim 18, further comprising a second end cap and/ora second focusing lens disposed between the partially reflective outputcoupler and the second optical fiber.
 20. The laser system of claim 17,further comprising a second end cap and/or a second focusing lensdisposed between the partially reflective output coupler and the secondoptical fiber.
 21. A laser system comprising: an array of beam emitterseach emitting a beam; focusing optics for focusing the beams toward adispersive element; a dispersive element for receiving and dispersingthe focused beams, thereby forming a multi-wavelength beam; an opticalfiber for receiving the multi-wavelength beam; and disposed opticallydownstream of the optical fiber, a partially reflective output couplerfor receiving the multi-wavelength beam, reflecting a first portionthereof back toward the dispersive element, and transmitting a secondportion thereof as an output beam composed of multiple wavelengths. 22.The laser system of claim 21, wherein the partially reflective outputcoupler is disposed within a laser processing head detachable from atleast a portion of the optical fiber.
 23. The laser system of claim 22,further comprising one or more focusing lenses disposed within the laserprocessing head.
 24. The laser system of claim 21, further comprisingone or more focusing lenses disposed optically downstream of the opticalfiber and optically upstream of the partially reflective output coupler.